omap: pm-debug: Enable wakeup_timer_milliseconds debugfs entry

Commit 8e2efde9 added milliseconds suspend wakeup time support but
same interface is not exported through debugfs

This patch enables the debugfs hook for wakeup_timer_milliseconds

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>